The production of sulfuric acid in mines and its entry into natural water sources create an ecological and economic problem. This "acid mine drainage" can have devastating effects on any water source that it enters.
During active mining operations, acid mine drainage can be controlled with some difficulty by programs utilizing neutralizing hydroxides. This treatment, however, is not easily applied after the mine site is no longer operational as acid mine drainage from former mining areas, coal refuse areas, waste-rock dumps and tailings becomes a serious problem. As the pH of a given waterway drops below 6, sensitive biota are affected. At a pH below 4, the stream will be virtually devoid of life. Further, mine acid will readily attack coexisting sulfide species and cause rapid dissolution of these minerals and release toxic concentrations of heavy metals.
Mine acid is formed under appropriate conditions by the oxidation of base metal sulfides. Microorganisms play an important role in catalyzing the oxidation of ferrous iron. The genus Thiobacillus is capable of oxidizing sulfide, thiosulfate, or elemental sulfur to sulfate and is further capable of oxidizing ferrous iron to ferric iron. Optimal pH levels for such oxidations are below 5 and generally in the area of 2.5.
When one refers to iron-oxidizing bacteria or sulfur-oxidizing bacteria, one is describing the mode of energy generation for the organisms, i.e., by oxidation of reduced iron or sulfur compounds. These types of bacteria obtain their carbon for
cell growth and reproduction from CO.sub.2. Thus the organisms are called chemolithotrophs, or, simply, autotrophs. Thiobacillus ferrooxidans is active in the pH range from about 2.0 to about 5.0, and is responsible for the production of significant amounts of sulfuric acid from mine tailings and waste rock dumps.
The conditions of an out of production mine provide all the requirements for the growth and reproduction of Thiobacillus ferrooxidans. These requirements include sources of sulfur compounds, ferrous ion, carbon dioxide, oxygen, and a low pH (&lt; 5) water supply. The organism will thrive in this environment by growing in the crevices and niches found in the walls of the mine. An inadequate supply of gases will limit the bacteria's growth in standing mine water and in refuse piles. Seepage water trickling down the walls of the mine will not prevent growth as it does not form a barrier thick enough to inhibit gas exchange.
Many efforts have been made in order to abate the acid mine drainage problem. These included attempts at sealing mines and using reducing atmospheres, which all failed for a variety of reasons. For these reasons there is great interest in finding a method to reduce acid production in mines.
Acrolein is a known pesticide that is used to treat liquids containing slime-forming microorganisms. Acrolein has been found to effectively control bacteria, such as Bacillus subtilis, Pseudomonas putrefaciens and Escherichia coli; fungi such as Penicillium italicum, Saccharomyces cereviseae and Helminthosporium turcicum; algae; macroinvertebrates, such as snails and clams; and aquatic plants and weeds. Acrolein is also more effective than other biocides, such as chlorine, in controlling macroinvertebrates and submerged, as well as floating, aquatic weeds and algae.
From an environmental point of view, acrolein is a good biocide because it is effective, detoxified readily and inexpensively, and is non-persistent. Water solutions of acrolein are readily and conveniently neutralized for disposal with sodium bisulfite. This reaction produces a non-toxic water-soluble salt. Acrolein is also neutralized by reacting with materials present in natural waters and is therefore self-neutralizing.
Acrolein has advantages over chlorine, a common biocide used in many aqueous systems. Unlike chlorine, acrolein is less reactive with oxidizable materials or other chemical constituents usually found in both surface and well water supplies. Chlorination alone is often uneconomical for pest control in systems using waters with high chlorine demands, or in systems heavily contaminated by process leakage. In addition, chlorine is frequently not very effective against filamentous algae, bacteria and/or shellfish in heavily contaminated systems.
Thus, acrolein possesses an effective ability to kill Thiobacillus ferrooxidans and not merely inhibit the bacteria's growth. Its vapor pressure is of the nature that will allow it to enter the crevices and niches in a mine in the gas phase. Further, in the gas phase acrolein is effective at low concentrations. This, along with the fact that it is detoxified readily and easily neutralized, made acrolein an ideal compound to investigate for controlling mine acid production and drainage.